Most modern theaters use projection systems which are operated by, or at least monitored by, some form of automated control system. One aspect of such a system is a means by which the monitor senses cues that a theater operator has placed on a film being shown such that peripheral equipment is properly turned on and off at the proper time and in the proper sequence. One method of detecting the cues is to simply have a pair of rollers with the electrical connection between the rollers made by a metallic strip placed on the film, i.e., the cue. This allows the use of a method involving the use of proximity sensors. One of the most successful proximity sensors involves the use of eddy current killed oscillation (ECKO) type proximity sensors. This type of sensor changes electrical state when a piece of metal passes relatively near a sensitive area, with no physical contact being required. Theaters typically use a cue which takes the form of an adhesive backed aluminum foil patch placed directly on the movie film. In addition to cue detection, film cue detectors normally include a pair of rollers which ride on opposite edges of the film so that if the film breaks, one or both of the rollers will drop, thereby tripping a cut-off switch.
Different movie theaters use different locations for placing cues on the film, i.e., the right or left edge or in the middle, between frames.
The proximity sensor must be positioned such that the metallic cue passes in close proximity to the sensor. This necessitates that the sensor be placed not only closely adjacent to the film's surface, but also transversely across the film depending on where the cue has been placed. This creates an inventory and production problem in producing a cue detector that would function properly independent of where the cues have been placed on the film. More recently, it has been desirable to have the cue detector perform additional functions in terms of automating the theater, such as operating house lights, audio equipment and the like. Hence, it is desirable to be able to use closely spaced proximity sensors operating simultaneously to detect cues placed on the left edge, right edge, and at other locations transversely across the film such as in the space between the individual picture frames. This desire in turn created a problem because conventional ECKO type proximity sensors cannot be placed directly adjacent to each other since their electrical fields would interfere with each other causing one or both sensors to malfunction.
As a safety feature, the film break detection switches, which in the past have been rollers riding on the film, ensure that film is still in the projector and has not broken or split. A roller type switch must be in physical contact with the film and can thereby damage it or become inoperable due to dirt, wear or corrosion.
It has also been deemed desirable to have a detector which would determine whether the film in the projector is moving or stationary. Another complication is that some projectors are capable of handling both 70 mm and 35 mm movie film.